1. Technical Field
The invention relates to a fuel injection pump for use with an internal combustion engine. The fuel injection pump includes a pump piston guided in a cylinder and having an adjustable stroke, a valve located adjacent an axial end of the pump piston between a cylinder chamber and an injection line and having a valve body which cooperates with the piston pump when the piston pump at a top dead center position, and a device for setting the stroke of the piston pump.
2. Description of the Prior Art
In fuel injection pumps in which the pump piston, at top dead center, cooperates with a valve, the pump piston effects the interruption of the injection process by actuating a valve body. A fuel injection pump of this kind is known from German Disclosure No. 31 00 725 A1. In this publication, especially in connection with FIG. 7, a fuel injection pump is described which has a relief valve actuated by the pump piston. In the fuel injection pump, a fuel chamber is located above a cylinder chamber and is connected through a fuel channel with the cylinder chamber. The relief valve is located parallel to the fuel channel and closes a passage from the fuel chamber into a return line for the fuel delivery system. A valve stem connected with the valve is guided in an upper portion of the cylinder chamber and, at the top dead center position of the pump piston, is in contact with the pump piston. The valve with the valve stem is biased by a spring against a valve seat, that is, in a direction toward the upper portion of the cylinder chamber. At the upper rim of the fuel chamber, located away from the cylinder chamber, is arranged a connection bore which leads into an injection line. The pump piston is driven by corresponding devices, as described also in the publication. During the stroke of the pump piston, the fuel is compressed in the cylinder chamber and forced through the fuel channel into the fuel chamber, and from there into the injection line. On reaching the desired injection pressure, the injection nozzles are released in the known way, and the injection process into the cylinder of the internal combustion engine begins. Before reaching the top dead center position, the end surface of the pump piston touches the end of the valve stem and presses against the relief valve. In this way, a passage between the fuel chamber and a return line is opened, and the pressure in the cylinder chamber, the fuel chamber and the injection line is relieved. As a result of the reduction in pressure, the injection nozzle is closed and the injection process ceases.
In fuel injection pumps which operate at high pressures, for example up to 2500 bar, the forces acting on the pump piston and the relief valve during the injection stroke are very high. Even the final speed of the piston before reaching the top dead center position may be relatively high. At the moment when the pump piston strikes against the stem of the relief valve, therefore, very high surface loads occur between the contact surfaces, which in a short time destroy the contact surfaces and impair the function of the device. Because of the sudden reduction of pressure on the surface of the relief valve, there is also the danger that the pump piston and the relief valve will be shot upward, and that there will be further damage to the pump piston, cylinder chamber and valve arrangements. To prevent this, large hold-back springs must be placed above the relief valve that such valve arrangements can hardly be designed to be controlled mechanically. Suitable arrangements can only be obtained if the carrying speed and also the pumping pressure are considerably reduced and, thus, the forces acting between the pump piston and the relief valve are also less. But reduction of the carrying speed brings known disadvantages, such as large pump pistons and the associated greater leakage, as well as poorer possibility of modulating the speed course of the piston. Lower injection pressures give a poorer atomization of the fuel in the internal combustion engine, and thus a later end of the combustion process. The known device has other disadvantages, since in the area of the relief valve and in the additional fuel chamber, a suction valve must be arranged which makes possible the suction of fuel from the fuel feeding system. The whole arrangement of relief valve, suction valve and connection channel has the result that the upper portion of the pump cylinder must be designed asymmetrical. In this way, with the heating of the cylinder there is danger that the latter will be deformed asymmetrically and in this way the perfect course of movement of the pump piston in the cylinder chamber may be prevented. Also, the high pressures which occur lead to irregular deformations of the upper cylinder part with equal consequences to the pump piston.